If you’re making the most out of WordPress’s media gallery for adding images and auto-lovely thumbnails to your site, you may notice there’s no option for cropping your medium sized thumbs.
As of WordPress 3.0.2 and earlier, only the WordPress Thumbnail image property includes the functionality to crop the image down to the exact dimensions referenced in your Settings > Media page; whereas quite often you’ll need to crop down your medium sizes images too to keep them consistent throughout your website.
I’ve just come across this little snippet in the WordPress forums which may be useful to others.
Add the following code block to your Functions.php file:
// Add support for cropping default WordPress medium images -